Re: [IBIS-Users] IBIS Simulation algorithm

> I also observe that, even if I remove the V-T table and supply only the
> [Ramp] information, HSPICE is able to simulate the buffer model with almost
> the same accuracy. [I have 4 VT (2 Rise and 2 Fall) tables originally]

Originally, IBIS had no V-T tables, and it worked OK. They were added
in a later revision to improve the transient accuracy.

Even after they were added to the IBIS spec, many simulators did not
use them. (Hspice took a while to catch up.)

How or why does it work without V-T tables? For most buffers,
waveforms are largely determined by the interaction between the
transistors (by way of their drive strengths in their On state), and
the load. How a driver gets from Off to On, is far less important
than how it interacts with the load, as described by its I-V curves.

For many buffers, their transistors are switched between Off and On
states faster than their outputs slew from L to H or H to L, because
of that interaction with the load. That means they spend most of the
time either Off (I=0), or fully On (I described by I-V table).

Without V-T tables, the simulator transitions between one transistor
fully On, to the other transistor fully On, over some short period of
time. The Ramp data helps the simulator control how quickly it should
go from one state to the other.

Andy

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